1. Field of the Invention
This invention relates generally to a control system for a hybrid vehicle provided with a plurality of prime movers for running a vehicle. More particularly, to a control system for a hybrid vehicle, which has an electric oil pump for generating an oil pressure to set a torque capacity of a power transmission system.
The disclosure of Japanese Patent Application No. 2004-14316 filed on Jan. 22, 2004 including specification, drawings and claims is incorporated herein by reference in its entirety.
2. Discussion of the Related Art
One example of the hybrid vehicle equipped with a “mechanical distribution type drive unit” is disclosed in Japanese Patent Laid-Open No. 2002-225578, and a construction thereof will be described briefly hereafter. In the disclosed hybrid vehicle, an engine torque is inputted to a carrier of a single pinion type planetary gear mechanism which constitutes a distributing mechanism, a first motor generator is connected to a sun gear, and an output member such as a counter drive gear etc. is connected to a ring gear. A second motor generator is connected to the output member or the ring gear through a transmission. The transmission is capable of switching a gear stage between a direct gear stage in which the entire transmission rotates integrally, and a low gear stage in which an output speed is lower than an input speed. Those gear stages are set by properly operating an engagement mechanism operated by the oil pressure.
The hybrid vehicle of this kind can be run not only by a motive power of an engine and the first motor generator, but also by using a torque outputted from the second motor generator as an assist torque, or only by an output torque of the second motor generator.
In order to ensure the oil pressure when the engine is halted, in the hybrid vehicle disclosed in Japanese Patent Laid-Open No. 2002-225578, it is conceivable to further provide an electric hydraulic pump which is capable of generating the oil pressure even when the engine is halted, in addition to a hydraulic pump which is driven by the engine. In this hybrid vehicle, moreover, the engine is connected to the first motor generator through a planetary gear mechanism, so that the engine can be started by carrying out a cranking (or a motoring) by the first motor generator. In this case, since an output shaft is connected to the planetary gear mechanism in addition to the engine and the first motor generator, torque acts on the output shaft in the direction to rotate it backward when cranking the engine by the first motor generator. For this reason, a “backward torque”, which appears on the output shaft when carrying out a cranking by the first motor generator is countervailed by outputting the torque to the output shaft by the second motor generator. As a result of this, it is possible to prevent a vibration in the vehicle and a backward movement of the vehicle.
On the other hand, the transmission is arranged between the second motor generator and the output shaft. In order to transmit the torque from the second motor generator to the output shaft, therefore, the transmission has to have a predetermined torque capacity. In this case, the engine has not yet been started so that it is impossible to obtain the oil pressure from the oil pump driven by the engine. Accordingly, it is necessary to generate the oil pressure by operating an electric oil pump provided in addition to the oil pump driven by the engine. As a result of this, both the first motor generator for starting the engine and the electric oil pump are driven at a starting time of the engine. If both of those first motor generator and electric oil pump are driven simultaneously, a load on an accumulator device such as a battery increases. This results in a shortage of electric power supply to the first motor generator or the like. Consequently, a cranking torque becomes insufficient so that it may take long time to startup the engine.